This invention relates to the abrading of workpiece surfaces, and more particularly to high precision lapping of magnetic transducing heads, utilizing a lapping surface and an abrasive liquid or slurry.
Throughout the broad range of data storage products, one constant seems to be the ever present challenge to increase the density of stored data. This allows storage of more data on a given surface area, and leads to product miniaturization, resulting in strict tolerances for linear dimensions and planarity measured in micro inches (millionths of an inch). Such tolerances call for machining processes and devices capable of meeting them. U.S. Pat. No. 4,270,316 (Kramer) discloses a process for polishing semiconductor discs, in which elastic bodies are positioned between the pressure piston and the back of the carrier plate in order to reduce the variance in pressure transmission between these members, resulting in a more uniform thickness throughout the disc. In U.S. Pat. No. 4,256,535 (Banks), a thin layer of water is interposed between a substrate and a semiconductor wafer mounted on the substrate, prior to positioning the substrate and wafer on a rotating polishing pad.
An apparatus for burnishing the magnetic oxide coating on flexible discs is disclosed in U.S. Pat. No. 4,347,689 (Hammond). A head supports a burnishing tape for movement relative to the surface being polished. A deformable cotton swab is contained in a cylindrical slot in the head, with an exposed portion of the swab elastically deformed and contacting the tape. Further, U.S. Pat. No. 4,459,781 (Li) discloses a grinding or polishing apparatus in which mixed abrasive particles, through either screen printing or a centrifugal process, are formed in concentric rings of decreasing particle size toward the center of an abrading wheel. Then, more refined polishing is accomplished simply by moving a workpiece radially inward relative to the wheel.
Magnetic transducing heads, used for example with rotatable magnetic discs in disc drives, are subject to especially fine manufacturing tolerances. Typical abrading equipment includes a lapping surface in which abrasive particles (e.g. diamond fragments) are embedded, and an abrasive slurry, e.g. a water soluble glycol base containing abrasive particles such as diamond fragments. As explained in U.S. Pat. No. 4,536,992 (Hennenfent et al), incorporated by reference as a part of this application, it is essential in thin film heads to control the throat height of the magnetic flux gap. Thin film transducers typically are formed by applying layers of an electrically conductive material (e.g. copper) and a magnetic flux conducting core or pole piece material encapsulated in aluminum oxide, along one side of a comparatively large body or slider, typically a ceramic material including aluminum oxide and titanium carbide. In use, the planar bottom surface of the slider is spaced vertically apart from a horizontal magnetic recording surface of the disc, supported by a film of air. The metallic layer, aluminum oxide and pole piece lie along a vertical edge of the slider, typically the trailing edge.
For increasing data density, the principal concerns include controlling and minimizing the vertical distance between the pole pieces (specifically the pole tips) and the magnetic recording surface. To this end, the slider bottom surface and pole tips should be, to the extent possible, co-planar. However, because the pole piece and immediately adjacent layers are more amenable to abrasive removal than the ceramic slider, normal lapping tends to more readily remove these layers, creating a problem known as pole tip recession. As a consequence of pole tip recession, the core material is separated from the recording surface at a distance greater than the slider bottom surface.
A related imperfection or deviation from co-planarity, is the unintentional rounding of the edge of the transducing head comprised of the core and adjacent layers. This rounding sometimes referred to as dubbing, occurs when the lapping surface is moved in a direction to encounter the transducing layers first. The curvature may or may not affect the height of the pole pieces, but it can undesirably influence the aerodynamics of the transducing head.
While pole tip recession and curvature can be reduced by slowing down the abrading rate, this has the unwanted side effect of substantially increasing machining time.
Therefore, it is an object of the present invention to provide a lapping process which achieves improved co-planarity without any substantial reduction in total abrasion time.
Another object of the invention is to provide an apparatus, using an abrasive slurry in conjunction with an abrasive lapping surface, for diverting the slurry away from a workpiece at selected times during abrasion.
Yet another object is to provide a retractable wiping apparatus, particularly suited for use with the precision lapping system disclosed in U.S. Pat. No. 4,536,992, positionable into a wiping engagement with the lapping plate at a selected time and just ahead of a workpiece, to guide abrasive slurry around and past the workpiece.